This invention relates to a mobile communication system and, in particular, to a method of selecting radio base stations in a mobile station of a code division multiple access (CDMA) cellular type mobile radio communication system in which the mobile station is simultaneously connected with the the radio base stations.
As is well known in the art, various multiple access types have been adapted in a mobile radio communication system. One of the multiple access type is a CDMA cellular type. The CDMA cellular type mobile radio communication system assigns to each channel with a particular code, transmits to the same repeater a modulated wave to which a carrier having the same carrier frequency is spectrum-spread with the code, establishes code synchronization in each receiving side, and identifies a desired channel. The CDMA cellular type mobile radio communication system may be called a SSMA (spread spectrum multiple access) cellular type mobile radio communication system.
The CDMA type mobile communication system comprises a plurality of mobile stations and a plurality of radio base stations each of which serves as a repeater. Each radio base station is called a base transceiver station in the art. In addition, each mobile station is referred to as a terminal. As described above, inasmuch as the plurality of mobile station carry out communication using the carrier with the same carrier frequency, it is necessary for the CDMA type mobile radio communication system to be uniform reception energy of an upward communication channel from each mobile station communicating with the radio base station without a position of the mobile station.
In order to be uniform the reception energy in the radio base station, the CDMA type mobile radio communication system carries out transmission power control for the upward communication channel as described in TIA (Telecommunication Industry Association)/EIA (Electronic Industries Association)/IS-95.
The CDMA cellular type mobile radio communication system comprises a plurality of radio base stations which are unevenly distributed and which simultaneously use a carrier having the same carrier frequency. In the CDMA cellular type mobile radio communication system, a mobile station must always connect with the radio base station having the minimum propagation loss and must suppress interference in other radio channels so as to minimize by carrying out transmission power control. Throughout the specification, the term xe2x80x9cconnectxe2x80x9d or xe2x80x9cconnectionxe2x80x9d means a state where a transmission path is established in order to carry out communication between the mobile station and the radio base station. Inasmuch as connection processing to a new radio base station takes a certain time, the radio base station having the minimum propagation loss is taken turns in dependency on variation of the propagation loss during this period and there is a case where a radio base station of a destination to be switched is affected by interference.
Description will proceed to the interference to the radio base station of the destination to be changed that occurs on connection processing to the above-mentioned new radio base station.
It is assumed that a mobile station connected to a first radio base station moves to a zone or a cell which is covered by a second radio base station. Under the circumstances, the mobile station starts switching processing for the radio base stations at a time when a difference between the propagation losses for the first and the second radio base stations is less than a connection threshold level. In addition, the mobile station carries out transmission power control for the first radio base station for a connection processing time interval.
When the propagation loss fox the second radio base station is less than that for the first radio base station during the connection processing time interval, the mobile station influences the interference for the second radio base station having a less propagation loss than that of the first base station. This is because the mobile station carries out the transmission power control for the first radio base station.
In order to overcome the above-mentioned interference problem due to this connection processing, a method is disclosed an article which is contributed by Allen Salmasi et al. to Proceeding in IEEE Vehicular Technology Conference, pages 57-62, May 1991, under the title of xe2x80x9cON THE SYSTEM DESIGN ASPECTS OF CODE DIVISION MULTIPLE ACCESS (CDMA) APPLIED TO DIGITAL CELLULAR AND PERSONAL COMMUNICATIONS NETWORKS.xe2x80x9d The method described in the above-mentioned reference is called a soft handoff or a soft handover. The soft handoff uses a technique which has ready for variation of the propagation loss by simultaneously connecting a mobile station with a plurality of radio base stations which include not only the radio base station having the minimum propagation loss but also a radio base station candidate which may come to have minimum pass loss hereafter.
Description will proceed to the soft handoff in a conventional CDMA cellular type mobile radio communication system. It is assumed that a mobile station connected to a first radio base station moves to a zone or a cell covered by a second radio base station. Under the circumstances, the mobile station starts connection processing for the second radio base station with connection of the first radio base station held at a time when a difference between the propagation losses for the first and the second radio base stations is less than a connection threshold level When connection processing for the second radio base station completes after the lapse of the connection processing time interval, connection of the first and the second radio base stations starts. Inasmuch as the transmission power control for the radio base station having the minimum propagation loss is always carried out during simultaneous connection, the issue related to the interference for the second radio base station dies although that issue does not die in a hard handoff method described above. Thereafter, connection of the first radio base station is released or disconnected at a time when the difference between the propagation losses of the first and the second radio base stations is more than a disconnection threshold level. In the manner which is described above, by carrying out the simultaneous connection of the radio base stations whose propagation loss levels are immediate, it is possible to cope with alternation of the radio base station having the minimum propagation loss and to suppress the interference.
The connection threshold level and the disconnection threshold level are set so that the latter is higher than the former. A difference between the connection threshold level and the disconnection threshold level is referred to as a hysteresis margin. That is, the hysteresis margin defines release from connection of the radio base stations. The hysteresis margin is prepared in order to prevent switching of connection and disconnection of the radio base stations from occurring frequently in a case where the difference between the propagation losses of the radio base stations varies heavily in the vicinity of the threshold level. The higher the connection threshold level and the disconnection threshold level become, the larger the number of the radio base stations simultaneously connected is.
However, a rate and a magnitude of variation in the propagation loss from the mobile station to each radio base station varies in accordance with a moving speed of the mobile station and circumferential propagation environment. For instance, the propagation loss extremely varies in cities where buildings are in close order so far as the mobile station moves slightly. This is because there are many objects which cut off paths for propagation of radio waves in the cities. In other words, in the cities, the variation of the propagation loss is fast and the magnitude thereof is large. On the other hand, in the suburbs of the cities, the variation of the propagation loss is slow and the magnitude thereof is small. This is because there are a few objects which cut off paths for propagation of radio waves in the suburbs.
In the above-mentioned conventional methods for selecting radio base stations, the threshold level for the propagation loss always has a constant level disregarding the circumferential propagation environment. However, it is necessary in an area where the variation of the propagation loss is fast such as the cities to set the threshold level for the propagation loss to be high in order to cope with rapid variation of the propagation loss. Otherwise, connection of the radio base station having the minimum propagation loss is late and it results in inflicting the interference upon the radio base station in question. On the other hand, in the suburbs where the variation of the propagation loss is slow, it is possible to set the threshold level for the propagation loss to be low. In other words, it is difficult to realize effective system operation in a case of using a fixed threshold level for the propagation loss. This is because excess and deficiency of the radio base stations simultaneously connected occur.
In addition, the hysteresis margin may be set to be small in a case where the variation of the propagation loss is slow. This is because the switching of the connection and the disconnection does not occur frequently. Conversely, if unnecessary large hysteresis margin is set, connection of useless radio base stations is maintained and it results in obstructing the effective system operation.
It is therefore an object of the present invention to provide a radio base station selecting method which is capable of realizing an effective system operation without excess and deficiency of the radio base stations simultaneously connected.
Other objects of this invention will become clear as the description proceeds.
According to an aspect of this invention, a method is for selecting radio base stations in a mobile station of a code division multiple access (CDMA) cellular type mobile radio communication system. The method comprises the steps of selecting the radio base stations to be connected so as to become more when a level of propagation loss changes rapidly, and of selecting the radio base stations to be connected so as to become less when the level of the propagation loss changes slowly.
According to another aspect of this invention, a method is for setting a hysteresis margin in a mobile station of a code division multiple access (CDMA) cellular type mobile radio communication system comprising a plurality of radio base stations. The hysteresis margin defines release from connection of said radio base stations. The method comprises the steps of setting the hysteresis margin so as to become larger when a level of propagation loss changes rapidly, and of setting the hysteresis margin so as to become smaller when the level of the propagation loss changes slowly.
According to still another aspect of this invention, a method is for selecting radio base stations in a mobile station of a code division multiple access (CDMA) cellular type mobile radio communication system, The method comprises the steps of selecting the radio base stations to be connected so as to become more when a moving speed of the mobile station is fast, and of selecting the radio base stations to be connected so as to become less when the moving speed of the mobile station is slow.
According to yet another aspect of this invention, a method is for setting a hysteresis margin in a mobile station of a code division multiple access (CDMA) cellular type mobile radio communication system comprising a plurality of radio base stations. The hysteresis margin defines release from connection of the radio base stations. The method comprises the steps of setting the hysteresis margin so as to become larger when a moving speed of the mobile station is fast, and of setting the hysteresis margin so as to become smaller when the moving speed of the mobile station is slow.
According to an aspect of this invention, a code division multiple access (CDMA) cellular type mobile radio communication system comprises a plurality of radio base stations and at least one mobile station. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from the mobile station to each of the radio base stations. Connected to the propagation loss measurement unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become more when a level of the propagation loss changes rapidly. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become less when the level of the propagation loss changes slowly.
According to another aspect of this invention, a code division multiple access (CDMA) cellular type mobile radio communication system comprises a plurality of radio base stations and at least one mobile station. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from the mobile station to each of the radio base stations. Connected to the propagation loss measurement unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss with reference to a connection threshold level and a disconnection threshold level which is higher than the connection threshold level by a hysteresis margin. The soft handoff radio base station selection unit sets the hysteresis margin so as to become larger when a level of the propagation loss changes rapidly. The soft handoff radio base station selection unit sets the hysteresis margin so as to become smaller when the level of the propagation loss changes slowly.
According to still another object of this invention, a code division multiple access (CDMA) cellular type mobile radio communication system comprises a plurality of radio base stations and at least one mobile station. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from the mobile station to each of the radio base stations and a moving speed detection unit for detecting a moving speed of the mobile station. Connected to the propagation loss measurement unit and the moving speed detection unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become more when the moving speed is fast. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become less when the moving speed is slow.
According to yet another object of this invention, a code division multiple access (CDMA) cellular type mobile radio communication system comprises a plurality of radio base stations and at least one mobile station. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from the mobile station to each of the radio base stations and a moving speed detection unit for detecting a moving speed of the mobile station. Connected to the propagation loss measurement unit and the moving speed detection unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss with reference to a connection threshold level and a disconnection threshold level which is higher than the connection threshold level by a hysteresis margin. The soft handoff radio base station selection unit sets the hysteresis margin so as to become larger when the moving speed is fast. The soft handoff radio base station selection unit sets the hysteresis margin so as to become smaller when the moving speed is slow.
According to an aspect of this invention, a mobile station is for use in a code division multiple access (CDMA) cellular type mobile radio communication system comprising a plurality of radio base stations. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from said mobile station to each of the radio base stations. Connected to the propagation loss measurement unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become more when a level of the propagation loss changes rapidly. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become less when the level of the propagation loss changes slowly.
According to another aspect of this invention, a mobile station is for use in a code division multiple access (CDMA) cellular type mobile radio communication system comprising a plurality of radio base stations. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from said mobile station to each of the radio base stations. Connected to the propagation loss measurement unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss with reference to a connection threshold level and a disconnection threshold level which is higher than the connection threshold level by a hysteresis margin. The soft handoff radio base station selection unit sets the hysteresis margin so as to become larger when a level of the propagation loss changes rapidly. The soft handoff radio base station selection unit sets the hysteresis margin so as to become smaller when the level of the propagation loss changes slowly.
According to still another aspect of this invention, a mobile station is for use in a code division multiple access (CDMA) cellular type mobile radio communication system comprising a plurality of radio base stations. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from the mobile station to each of the radio base stations and a moving speed detection unit for detecting a moving speed of the mobile station. Connected to the propagation loss measurement unit and the moving speed detection unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become more when the moving speed is fast. The soft handoff radio base station selection unit selects the radio base stations to be connected so as to become less when the moving speed is slow.
According to yet another aspect of this invention, a mobile station is for use in a code division multiple access (CDMA) cellular type mobile radio communication system comprising a plurality of radio base stations. The mobile station comprises a propagation loss measurement unit for measuring a propagation loss from the mobile station to each of the radio base stations and a moving speed detection unit for detecting a moving speed of the mobile station. Connected to the propagation loss measurement unit and the moving speed detection unit, a soft handoff radio base station selection unit determines the radio base stations to be connected or to be disconnected on the basis of the propagation loss with reference to a connection threshold level and a disconnection threshold level which is higher than the connection threshold level by a hysteresis margin. The soft handoff radio base station selection unit sets the hysteresis margin so as to become larger when the moving speed is fast. The soft handoff radio base station selection unit sets the hysteresis margin so as to become smaller when the moving speed is slow.